Question about Planetary Formation

[Widdekind]

According to the National Geographic Channel documentary Naked Science -- Journey to Jupiter (TV), Ganymede, Callisto, Io, & Europa are Jupiter's "four largest moons".

 

But, they are also the four innermost moons, yes ??

 

And, most basic textbooks observe that Jupiter's Moon System is much like a "mini-Star System". Thus, on the basis of this "Gas Giant analogy", the Sun's innermost planets ought to be the biggest planets.

 

 

 

QUESTION: Could it be, that the four Terrestrial Planets are actually the biggest planets — in particular, bigger than the rocky cores of the Gas Giants ??

 

And, on the strength of the same said analogy, why wouldn't Jupiter possess "outer moons" w/ large gaseous envelopes ??

 

 

 

According to the same said documentary, the Galileo spaceprobe discovered Argon, Krypton, Carbon & Nitrogen in Jupiter's atmosphere at 2-3x solar concentrations. And, since these gases all condense at considerably colder temperatures than those at the present position of that planet, possibly Jupiter migrated inwards from an initially greater distance from the Sun. [Wasn't the Sun some ~30% dimmer in the deep past ?]

 

<<I'm being booted off the computer>>

[insane_alien]

actually the four inner most moons are Metis, Adrastea, Amalthea and Thebe.

 

also, withthe inner system, a lot of the lighter particles and gases were just blown away from the sun leaving less stuff to make planets from. this didn't happen with jupiter.

[Klaynos]

http://berkeley.edu/news/media/releases/2008/11/25_core.shtml

 

Interesting read on gas giant cores.

[D H]

Stop being silly, Widdekind. Analogies, particularly those presented in elementary texts, are just that: analogies. Take them too far and you end up with nonsense.

[Moontanman]

I have to ask, how could there be ices inside jupiter when it is so hot?

[insane_alien]

pressure is how. there's lots and lots of pressure.

[SH3RL0CK]

In addition, I think it is worth noting that most planets of other star systems are gas giants very near the star. This is in contrast to our solar system where the gas giants are much further out, which does make me suspect our solar system might be somewhat "different".

 

However, the observation that most extrasolar planetary systems have large gas giants near the star is because these types of planetary systems are easiest to find. It will be some time before we will have an accurate survey of other planetary systems.

[Widdekind]

http://berkeley.edu/news/media/releases/2008/11/25_core.shtml

 

Interesting read on gas giant cores.

 

Thanks for the link

[Moontanman]

pressure is how. there's lots and lots of pressure.

 

I understand how something can be compressed, even compressed into a solid but does that make it ice? Water deep in the ocean super heated by touching molten lava is the same density as steam as it is as a liquid but it's still not liquid water. I'm not trying to be obtuse , just trying to understand.

[insane_alien]

well all ice is is the solid form of water (and there are several different types of ice depending on crystal structure).

 

so yes, something compressed to the point of being a solid is an ice. a fork is made of iron ice(fair enough with some carbon impurity, but still.).

 

with the hydrothermal vents you have super critical water. the phases of gas and liquid are no longer distinct and hence have no meaning. it is a super critical fluod then. the same phenomenon has not been observed between solids and liquids.

 

and for the record, its not a dumb question. it's actually quite a good one as it doesn't follow everyday experience.

[Edtharan]

In addition, I think it is worth noting that most planets of other star systems are gas giants very near the star. This is in contrast to our solar system where the gas giants are much further out, which does make me suspect our solar system might be somewhat "different".

 

However, the observation that most extrasolar planetary systems have large gas giants near the star is because these types of planetary systems are easiest to find. It will be some time before we will have an accurate survey of other planetary systems.

This could actually be a bit of selective reporting.

 

The way we have been detecting extra solar planets is by their gravitational effect on their parent star. The effect they look for is a wobbling of the star due to the planet orbiting it.

 

Needles to say, a planet that is far away form its star will not cause a large wobble tot ehir parent star, also when they are far out the rate of wobble will be much slower.

 

The way the measure the wobble of a star is by looking at the red/blue shiftas it wobbles. If this change is slow, then the background noise (of the star, gas between us and the star, terrestrial sources of interference, heat in the detectors, etc) will swamp any signs of variation in the red/blue shift of these stars, thus making any such planets undetectable.

 

So, the main reason that the main exoplanets that have been are these Hot Jupiters is because these are the most easy to spot.

 

There are ways of spotting planets that are far out from their stars by looking at the dimming of the star's light that reaches us as the planet transits the star. There are two things that make such events very rare:

 

First: If a planet is far out, then the orbital period is long, so the times when the planet will be directly between us and its parent star doesn't occur very often (eg: For Jupiter this is around 12 years), and several observations are needed to confirm the existence of the planet.

 

The second reason is that the orbital alignment of the planet must be that the planet does pass between us and its parent star. As there is no reason that a planet must be aligned this way, then this makes any such alignment purely accidental so even if there were planets around many stars, it might be that only a very few could be detected like this (and yet some have - which indicates that planets around stars are quite common).

 

Finally, the solar system is not as stable as one might think. Over cosmic time (hundreds of millions of years) planets within the solar system move around. 4.5 billions years (or so) ago, a planet the size of Mars was moving around the solar system so much that it hit Earth (and the debris from this formed the Moon). We are in a "fairly" stable period of the solar system, but that can (and is likely to) change. Some models predict that Mercury or Venus will eventually be flung out of the solar system entirely (into interstellar space), or collide with Earth in around 7 billion years (give or take a few billion years).

 

As planets, even in our own solar system, are known to move about, these Hot Jupiters could ahve formed further out and then moved in towards their star.

[Widdekind]

actually the four inner most moons are Metis, Adrastea, Amalthea and Thebe.

 

also, withthe inner system, a lot of the lighter particles and gases were just blown away from the sun leaving less stuff to make planets from. this didn't happen with jupiter.

 

(sorry for the late response / realization)

 

That's an important distinction, planetary formation is only-in-some-ways similar, to star formation. In particular, smaller planetary systems might actually create larger 'inner moon system' tides, than are present in inner star systems. For example, although our sun is ~1000x more massive than Jupiter, Mercury orbits ~450x farther from the sun, than Metis from Jupiter -- and tidal forces depend on D^-3. Perhaps, then, Jupiter thusly possesses an inner ring system, of tidally disrupted debris, whereas our sun does not.

 

And, so, that illustrates, that due to the critical presence of fusion power, present in stars but not planets, the 'temperature regime', in which planetary formation occurs, around stars, is dramatically different, than that of moons, around (gas giant) planets -- which determines the body's 'bulk composition', from available materials, and makes 'all the difference in the world / moon' (if you will).

[Ophiolite]

The four major satellites of Jupiter are probably the only original satellites. The others are captured asteroids.

[Moontanman]

The four major satellites of Jupiter are probably the only original satellites. The others are captured asteroids.

 

 

Do you have any info on that idea? It's new to me.

[Ophiolite]

Do you have any info on that idea? It's new to me.

I'll have to dig around. To my thinking it is old, established thought, almost self evident.

 

Digging done. It seems I was over (under?) enthusiastic. Jupiter has eight regular satellites, not the four I suggested. Regular satellites are ones with prograde orbits and minimal eccentricity. The four Galilean satellites are, as I noted, definitely regular, plus a further four. (See, for example,Origin and history of the Outer Planets, page 396.)

 

Attempts to explain the irregular satellites are still ongoing. For example, Nesvorny,D. et al Capture of Irregular Satellites During Planetary Encounters, The Astronomical Journal, May 2007. They note their proposed mechanism would not explain the origin of the Jovian irregulars, but implicitly accept these are captured asteroids (or small comets).

Edited April 27, 2011 by Ophiolite